Thermoplastic films, and in particular stretchable thermoplastic films have proven to be useful in the packaging of food as well as non-food items.
An example of a stretchable film particularly useful in the packaging of food products such as poultry is the film described in U. S. Patent No. 4,617,241 issued to Mueller and describing a stretch/shrink film. Other films, such as those described in U. S. Patent No. 4,399,180 issued to Briggs et al have proven especially useful in pallet wrapping or the wrapping of products prepackaged in a container.
While such films have been successful in certain applications, enhanced oxygen barrier properties are needed in the packaging of certain perishable food items such as cheese. This is required to maintain the quality of the product and maintain an aesthetically pleasing appearance for the customer at point of sale.
A polymeric material which has proven particularly effective in thermoplastic film technology has been vinylidene chloride copolymer commonly known as saran, a trademark of Dow Chemical Company The comonomers typically used in connection with a vinylidene chloride copolymer are vinyl chloride or methyl acrylate. While the oxygen transmission rates of saran are exceptionally low, therefore making this material attractive as a barrier polymer, saran of the type having no plasticizer or a low plasticizer content has a tendency to be brittle and non-stretchable when used in multilayer thermoplastic films. Because of this feature, multilayer films made with saran can be unacceptable in applications where a substantial amount of stretchability or elongation in the film is required to produce an acceptable package.
It is therefore desirable to combine the good oxygen barrier properties of copolymers such as saran or ethylene vinyl alcohol copolymer, with sufficient elongation properties to produce a film which has both stretch characteristics and oxygen barrier characteristics.
Of interest is U.S. Pat. No. 4,226,882 issued to Yoshikawa et al disclosing a five layer film having a core layer of polyvinylidene chloride, intermediate layers of ethylene copolymer, and outer layers of ionomer or, in one outer layer, an olefin.
Also of interest is U.S. Pat. No. 4,352,844 issued to Bornstein and disclosing a multilayer film in which a layer of vinylidene chloride copolymer is extruded with one or more layers of ethylene vinyl acetate copolymer (EVA).
U.S. Pat. No 4,376,799 issued to Tusim discloses a multilayer material having a polyvinylidene chloride core, combined with the use of ethylene vinyl acetate copolymer as a blending material in skin layers and intermediate layers of a five layer structure.
U.S. Pat. No. 4,390,587 issued to Yoshimura et al discloses multilayer film having a core layer of polyvinylidene chloride, intermediate layers of ethylene vinyl acetate copolymer, and outer layers of the same composition as the intermediate layers or, for one outer layer, an ionomer.
U.S. Pat. No. 4,457,960 issued to Newsome discloses the use of saran in combination with ethylene vinyl acetate copolymer.
An additional useful property in such applications is flexibility or lower modulus.
It is therefore an object of the present invention to provide a thermoplastic film with a combination of oxygen barrier and elongation properties.
It is a further object of the invention to provide a film for use in packaging applications which has a relatively low modulus i.e. relatively good flexibility.